M ay 2 00 2 Accepted for publication in ApJ The small - scale structure of the Magellanic Stream

We have mapped two regions at the northern tip of the Magellanic Stream in neutral hydrogen 21-cm emission using the Arecibo telescope. The new data are used to study the morphology and properties of the Stream far away from the Magellanic Clouds, as well as to provide indirect constraints on the properties of the Galactic Halo. We investigate confinement mechanisms for the Stream clouds and conclude that these clouds cannot be gravitationally confined or in free expansion. The most likely mechanism for cloud confinement is pressure support from the hot Galactic Halo gas. This allows us to place an upper limit on the Halo density: nh(15 kpc) = 10 −3 cm and/or nh(45 kpc) = 3× 10 −4 cm depending on the distance. These values are significantly higher than predicted for an isothermal stratified Halo.